Hitherto, components which are used in order to impart an X-ray opacity to dental restorative materials contain elements having a mass number equal to or higher than that of strontium. Oxides or sulfates of these elements have a high refractive index. In addition, in case of the sulfates, since they are crystalline with low symmetry, when they are used for a dental restorative material as a polycrystalline powder, their transparency is deteriorated. Accordingly, in case where esthetics is required, and translucency close to that in a tooth is to be obtained, their amount for use is restricted.
Also, fluorides of these elements have a refractive index lower than that of the oxides or sulfates (for example, the refractive index of ytterbium trifluoride is 1.53), the modifying effect of generally employed silane coupling agents is low, the mechanical characteristics are hardly realized. Accordingly, their amount for use is restricted.
For these reasons, components which are generally employed at present in order to impart an X-ray opacity are an oxide-glass containing barium, and a glass containing barium in an amount enough to have X-ray opacity (this glass being hereunder referred to as "barium oxide glass") has a refractive index of 1.53 or more. Such a barium oxide glass is combined with .alpha.-quartz (refractive index: 1.544 & 1.553) with no X-ray opacity, which had been used prior to the time when importance was attached to the X-ray opacity, and used as a dental restorative material.
Hitherto, as matrix monomers to be used to obtain a dental restorative material having translucency close to that of a tooth by using a such a barium oxide glass and .alpha.-quartz, since they are required to have a high refractive index, monomers or oligomers having a benzene ring in a molecular structure thereof, and particularly "bis-phenol A" (hereafter to be referred to as bisphenol A) in a molecule thereof were suitably used. Besides, as monomers for making the viscosity low and having a high refractive index, mono-(meth)acryloyl monomers such as phenyl (meth)acrylate, phenoxyethyl (meth)acrylate, and benzyl (meth)acrylate were used.
In recent years, some of the chemical substances which are generally employed are pointed out to cause an action against living bodies similar to that of hormones (these chemical substances being called as "environmental hormones" or "environmental endocrine disruptors, etc."). Of these substances, there are chemical substances having a benzene ring, and nonyl phenol which is formed upon decomposition of a surfactant or bisphenol A which is generally employed in epoxy resins, etc. are exemplified as a causative substance thereof. Of monomers or oligomers which are generally employed in the dental field, there are ones obtained by modifying bisphenol A with epichlorohydrin, etc. and (meth)acryloyl a terminal end(s) thereof. Since such monomers or oligomers are purified so that the bisphenol A is not substantially eluted out, but a pseudohormonal action takes place in an extremely low concentration, so the elution of the bisphenol A from these monomers or oligomers becomes a problem.
In the light of the above, in these conventional technologies used monomers having a benzene ring, in order to obtain a dental restorative material having an X-ray opacity and having translucency close to that of a tooth, there was a fear of the action as an environmental hormone.
On the other hand, in order to give translucency close to that of a natural tooth to a dental restorative material, the adjusting of a refractive index between a matrix monomer and a filler is carried out.
The inventions as disclosed in Japanese Patent Publication No. 4-48801(1992) and Japanese Patent Publication No. 7-45373(1995) are concerned with a technology in which while paying an attention to the difference in refractive index, a fluctuation of the translucency of a dental restorative material due to the change in refractive index by curing of a matrix monomer is intended to make small. The invention as disclosed in Japanese Patent Laid-Open No. 9-169613(1997) is concerned with a technology in which it is an index for the translucency to define a degree of diffusion from the angle reliance of a transmitted light. However, even components which are satisfied with these requirements are insufficient in esthetics from the clinical standpoint, and a deviation in hue is generated due to the thickness of a dental restorative material. One of the causes is one by light scattering of colloidal silica having a mean particle size of from 20 to 70 nm, in which a so-called opal-effect to be generated by the fact that the visible light in a short wavelength is scattered, while the visible light in a long wavelength is transmitted. This opal-effect is slightly observed on human teeth and hence, it is considered to be necessary to properly impart it to a dental restorative material. However, if a volume fraction of colloidal silica in the dental restorative material is too high, the hue becomes strong in red at the portion of a thin filling layer, while the hue becomes strong in blue, and the lightness is high at the portion of a thin filling layer. That is, the hue relies on the thickness of the filling layer thereof. Even when the colloidal silica is eliminated, there is generated a tendency in which the hue relies on the thickness of the filling layer in compounding in the examples as disclosed in Japanese Patent Publication No. 4-48801(1992) and Japanese Patent Publication No. 7-45373(1995). In other words, when not only the refractive index in a D-line of sodium of an organic material such as the matrix monomer exceeds that of an inorganic material such as colloidal silica, but also the volume fraction of said inorganic materials is large, opal-effect like optical characteristics are realized, and the hue is liable to rely on the thickness of the filling layer. In particular, when the filling layer is thick, the hue becomes bluish, and such fails in the esthetics from the clinical standpoint.